The occurrence of historical facts is said to be demonstrated, when
the evidence that they happened is of such a character as to render the
assumption that they did not happen in the highest degree improbable;
and the question I now have to deal with is, whether evidence in favour
of the evolution of animals of this degree of cogency is, or is not,
obtainable from the record of the succession of living forms which is
presented to us by fossil remains.
Those who have attended to the progress of palaeontology are aware that
evidence of the character which I have defined has been produced in
considerable and continually-increasing quantity during the last few
years. Indeed, the amount and the satisfactory nature of that evidence
are somewhat surprising, when we consider the conditions under which
alone we can hope to obtain it.
It is obviously useless to seek for such evidence except in localities
in which the physical conditions have been such as to permit of the
deposit of an unbroken, or but rarely interrupted, series of strata
through a long period of time; in which the group of animals to be
investigated has existed in such abundance as to furnish the requisite
supply of remains; and in which, finally, the materials composing the
strata are such as to ensure the preservation of these remains in a
tolerably perfect and undisturbed state.
It so happens that the case which, at present, most nearly fulfils
all these conditions is that of the series of extinct animals which
culminates in the horses; by which term I mean to denote not merely the
domestic animals with which we are all so well acquainted, but their
allies, the ass, zebra, quagga, and the like. In short, I use "horses"
as the equivalent of the technical name _Equidae,_ which is applied to
the whole group of existing equine animals.
The horse is in many ways a remarkable animal; not least so in the fact
that it presents us with an example of one of the most perfect pieces
of machinery in the living world. In truth, among the works of human
ingenuity it cannot be said that there is any locomotive so perfectly
adapted to its purposes, doing so much work with so small a quantity
of fuel, as this machine of nature's manufacture--the horse. And, as
a necessary consequence of any sort of perfection, of mechanical
perfection as of others, you find that the horse is a beautiful
creature, one of the most beautiful of all land-animals. Look at the
perfect balance of its form, and the rhythm and force of its action. The
locomotive machinery is, as you are aware, resident in its slender fore
and hind limbs; they are flexible and elastic levers, capable of being
moved by very powerful muscles; and, in order to supply the engines
which work these levers with the force which they expend, the horse
is provided with a very perfect apparatus for grinding its food and
extracting therefrom the requisite fuel.
Without attempting to take you very far into the region of osteological
detail, I must nevertheless trouble you with some statements respecting
the anatomical structure of the horse; and, more especially, will it be
needful to obtain a general conception of the structure of its fore and
hind limbs, and of its teeth. But I shall only touch upon those points
which are absolutely essential to our inquiry.
Let us turn in the first place to the fore-limb. In most quadrupeds, as
in ourselves, the fore-arm contains distinct bones called the radius
and the ulna. The corresponding region in the horse seems at first
to possess but one bone. Careful observation, however, enables us to
distinguish in this bone a part which clearly answers to the upper end
of the ulna. This is closely united with the chief mass of the bone
which represents the radius, and runs out into a slender shaft which may
be traced for some distance downwards upon the back of the radius, and
then in most cases thins out and vanishes. It takes still more trouble
to make sure of what is nevertheless the fact, that a small part of the
lower end of the bone of the horse's fore arm, which is only distinct in
a very young foal, is really the lower extremity of the ulna.
What is commonly called the knee of a horse is its wrist. The "cannon
bone" answers to the middle bone of the five metacarpal bones, which
support the palm of the hand in ourselves. The "pastern," "coronary,"
and "coffin" bones of veterinarians answer to the joints of our middle
fingers, while the hoof is simply a greatly enlarged and thickened nail.
But if what lies below the horse's "knee" thus corresponds to the
middle finger in ourselves, what has become of the four other fingers or
digits? We find in the places of the second and fourth digits only two
slender splint-like bones, about two-thirds as long as the cannon bone,
which gradually taper to their lower ends and bear no finger joints, or,
as they are termed, phalanges. Sometimes, small bony or gristly nodules
are to be found at the bases of these two metacarpal splints, and it
is probable that these represent rudiments of the first and fifth toes.
Thus, the part of the horse's skeleton, which corresponds with that of
the human hand, contains one overgrown middle digit, and at least two
imperfect lateral digits; and these answer, respectively, to the third,
the second, and the fourth fingers in man.
Corresponding modifications are found in the hind limb. In ourselves,
and in most quadrupeds, the leg contains two distinct bones, a large
bone, the tibia, and a smaller and more slender bone, the fibula. But,
in the horse, the fibula seems, at first, to be reduced to its upper
end; a short slender bone united with the tibia, and ending in a point
below, occupying its place. Examination of the lower end of a young
foal's shin bone, however, shows a distinct portion of osseous matter,
which is the lower end of the fibula; so that the apparently single,
lower end of the shin bone is really made up of the coalesced ends of
the tibia and fibula, just as the apparently single, lower end of the
fore-arm bone is composed of the coalesced radius and ulna.
The heel of the horse is the part commonly known as the hock. The hinder
cannon bone answers to the middle metatarsal bone of the human foot, the
pastern, coronary, and coffin bones, to the middle toe bones; the hind
hoof to the nail; as in the fore-foot. And, as in the fore-foot, there
are merely two splints to represent the second and the fourth toes.
Sometimes a rudiment of a fifth toe appears to be traceable.
The teeth of a horse are not less peculiar than its limbs. The living
engine, like all others, must be well stoked if it is to do its work;
and the horse, if it is to make good its wear and tear, and to exert the
enormous amount of force required for its propulsion, must be well and
rapidly fed. To this end, good cutting instruments and powerful and
lasting crushers are needful. Accordingly, the twelve cutting teeth of a
horse are close-set and concentrated in the fore-part of its mouth, like
so many adzes or chisels. The grinders or molars are large, and have an
extremely complicated structure, being composed of a number of different
substances of unequal hardness. The consequence of this is that they
wear away at different rates; and, hence, the surface of each grinder is
always as uneven as that of a good millstone.
I have said that the structure of the grinding teeth is very
complicated, the harder and the softer parts being, as it were,
interlaced with one another. The result of this is that, as the tooth
wears, the crown presents a peculiar pattern, the nature of which is
not very easily deciphered at first; but which it is important we should
understand clearly. Each grinding tooth of the upper jaw has an _outer
wall_ so shaped that, on the worn crown, it exhibits the form of two
crescents, one in front and one behind, with their concave sides turned
outwards. From the inner side of the front crescent, a crescentic _front
ridge_ passes inwards and backwards, and its inner face enlarges into a
strong longitudinal fold or _pillar._ From the front part of the hinder
crescent, a _back ridge_ takes a like direction, and also has its
_pillar._
The deep interspaces or _valleys_ between these ridges and the outer
wall are filled by bony substance, which is called _cement,_ and coats
the whole tooth.
The pattern of the worn face of each grinding tooth of the lower jaw is
quite different. It appears to be formed of two crescent-shaped ridges,
the convexities of which are turned outwards. The free extremity of each
crescent has a _pillar,_ and there is a large double _pillar_ where
the two crescents meet. The whole structure is, as it were, imbedded in
cement, which fills up the valleys, as in the upper grinders.
If the grinding faces of an upper and of a lower molar of the same
side are applied together, it will be seen that the opposed ridges are
nowhere parallel, but that they frequently cross; and that thus, in the
act of mastication, a hard surface in the one is constantly applied to
a soft surface in the other, and _vice versa._ They thus constitute a
grinding apparatus of great efficiency, and one which is repaired as
fast as it wears, owing to the long-continued growth of the teeth.
Some other peculiarities of the dentition of the horse must be noticed,
as they bear upon what I shall have to say by and by. Thus the crowns
of the cutting teeth have a peculiar deep pit, which gives rise to the
well-known "mark" of the horse. There is a large space between the
outer incisors and the front grinder. In this space the adult male horse
presents, near the incisors on each side, above and below, a canine or
"tush," which is commonly absent in mares. In a young horse, moreover,
there is not unfrequently to be seen in front of the first grinder, a
very small tooth, which soon falls out. If this small tooth be counted
as one, it will be found that there are seven teeth behind the canine
on each side; namely, the small tooth in question, and the six great
grinders, among which, by an unusual peculiarity, the foremost tooth is
rather larger than those which follow it.
I have now enumerated those characteristic structures of the horse which
are of most importance for the purpose we have in view.
To any one who is acquainted with the morphology of vertebrated animals,
they show that the horse deviates widely from the general structure
of mammals; and that the horse type is, in many respects, an extreme
modification of the general mammalian plan. The least modified mammals,
in fact, have the radius and ulna, the tibia and fibula, distinct and
separate. They have five distinct and complete digits on each foot, and
no one of these digits is very much larger than the rest. Moreover,
in the least modified mammals, the total number of the teeth is very
generally forty-four, while in horses, the usual number is forty, and in
the absence of the canines, it may be reduced to thirty-six; the incisor
teeth are devoid of the fold seen in those of the horse: the grinders
regularly diminish in size from the middle of the series to its front
end; while their crowns are short, early attain their full length, and
exhibit simple ridges or tubercles, in place of the complex foldings of
the horse's grinders.
Hence the general principles of the hypothesis of evolution lead to the
conclusion that the horse must have been derived from some quadruped
which possessed five complete digits on each foot; which had the
bones of the fore-arm and of the leg complete and separate; and which
possessed forty-four teeth, among which the crowns of the incisors and
grinders had a simple structure; while the latter gradually increased
in size from before backwards, at any rate in the anterior part of the
series, and had short crowns.
And if the horse has been thus evolved, and the remains of the different
stages of its evolution have been preserved, they ought to present
us with a series of forms in which the number of the digits becomes
reduced; the bones of the fore-arm and leg gradually take on the equine
condition; and the form and arrangement of the teeth successively
approximate to those which obtain in existing horses.
Let us turn to the facts, and see how far they fulfil these requirements
of the doctrine of evolution.
In Europe abundant remains of horses are found in the Quaternary and
later Tertiary strata as far as the Pliocene formation. But these
horses, which are so common in the cave-deposits and in the gravels of
Europe, are in all essential respects like existing horses. And that is
true of all the horses of the latter part of the Pliocene epoch. But, in
deposits which belong to the earlier Pliocene and later Miocene epochs,
and which occur in Britain, in France, in Germany, in Greece, in India,
we find animals which are extremely like horses--which, in fact, are so
similar to horses, that you may follow descriptions given in works upon
the anatomy of the horse upon the skeletons of these animals--but which
differ in some important particulars. For example, the structure of
their fore and hind limbs is somewhat different. The bones which, in the
horse, are represented by two splints, imperfect below, are as long
as the middle metacarpal and metatarsal bones; and, attached to the
extremity of each, is a digit with three joints of the same general
character as those of the middle digit, only very much smaller. These
small digits are so disposed that they could have had but very little
functional importance, and they must have been rather of the nature of
the dew-claws, such as are to be found in many ruminant animals. The
_Hipparion,_ as the extinct European three-toed horse is called, in
fact, presents a foot similar to that of the American _Protohippus_
(Fig. 9), except that, in the _Hipparion,_ the smaller digits are
situated farther back, and are of smaller proportional size, than in the
_Protohippus._
The ulna is slightly more distinct than in the horse; and the whole
length of it, as a very slender shaft, intimately united with the
radius, is completely traceable. The fibula appears to be in the same
condition as in the horse. The teeth of the _Hipparion_ are essentially
similar to those of the horse, but the pattern of the grinders is in
some respects a little more complex, and there is a depression on the
face of the skull in front of the orbit, which is not seen in existing
horses.
In the earlier Miocene, and perhaps the later Eocene deposits of some
parts of Europe, another extinct animal has been discovered, which
Cuvier, who first described some fragments of it, considered to be a
_Palaeotherim._ But as further discoveries threw new light upon its
structure, it was recognised as a distinct genus, under the name of
_Anchitherium._
In its general characters, the skeleton of _Anchitherium_ is very
similar to that of the horse. In fact, Lartet and De Blainville called
it _Palæotherium equinum_ or _hippoides;_ and De Christol, in 1847, said
that it differed from _Hipparion_ in little more than the characters of
its teeth, and gave it the name of _Hipparitherium._ Each foot
possesses three complete toes; while the lateral toes are much larger in
proportion to the middle toe than in _Hipparion,_ and doubtless rested
on the ground in ordinary locomotion.
The ulna is complete and quite distinct from the radius, though firmly
united with the latter. The fibula seems also to have been complete. Its
lower end, though intimately united with that of the tibia, is clearly
marked off from the latter bone.
There are forty-four teeth. The incisors have no strong pit. The canines
seem to have been well developed in both sexes. The first of the seven
grinders, which, as I have said, is frequently absent, and, when it does
exist, is small in the horse, is a good-sized and permanent tooth, while
the grinder which follows it is but little larger than the hinder ones.
The crowns of the grinders are short, and though the fundamental pattern
of the horse-tooth is discernible, the front and back ridges are
less curved, the accessory pillars are wanting, and the valleys, much
shallower, are not filled up with cement.
Seven years ago, when I happened to be looking critically into the
bearing of palaentological facts upon the doctrine of evolution, it
appeared to me that the _Anchitherium,_ the _Hipparion,_ and the modern
horses, constitute a series in which the modifications of structure
coincide with the order of chronological occurrence, in the manner in
which they must coincide, if the modern horses really are the result
of the gradual metamorphosis, in the course of the Tertiary epoch, of a
less specialised ancestral form. And I found by correspondence with the
late eminent French anatomist and palaeontologist, M. Lartet, that he
had arrived at the same conclusion from the same data.
That the _Anchitherium_ type had become metamorphosed into the
_Hipparion_ type, and the latter into the _Equine_ type, in the course
of that period of time which is represented by the latter half of the
Tertiary deposits, seemed to me to be the only explanation of the facts
for which there was even a shadow of probability. [3]
And, hence, I have ever since held that these facts afford evidence of
the occurrence of evolution, which, in the sense already defined, may be
termed demonstrative.
All who have occupied themselves with the structure of _Anchitherium,_
from Cuvier onwards, have acknowledged its many points of likeness to a
well-known genus of extinct Eocene mammals, _Palaeotherium._ Indeed, as
we have seen, Cuvier regarded his remains of _Anchitherium_ as those of
a species of _Palaeotherium._ Hence, in attempting to trace the pedigree
of the horse beyond the Miocene epoch and the Anchitheroid form, I
naturally sought among the various species of Palaeotheroid animals
for its nearest ally, and I was led to conclude that the _Palaeotherium
minus (Plagiolophus)_ represented the next step more nearly than any
form then known.
I think that this opinion was fully justifiable; but the progress of
investigation has thrown an unexpected light on the question, and has
brought us much nearer than could have been anticipated to a knowledge
of the true series of the progenitors of the horse.
You are all aware that, when your country was first discovered by
Europeans, there were no traces of the existence of the horse in any
part of the American Continent. The accounts of the conquest of Mexico
dwell upon the astonishment of the natives of that country when they
first became acquainted with that astounding phenomenon--a man seated
upon a horse. Nevertheless, the investigations of American geologists
have proved that the remains of horses occur in the most superficial
deposits of both North and South America, just as they do in Europe.
Therefore, for some reason or other--no feasible suggestion on that
subject, so far as I know, has been made--the horse must have died out
on this continent at some period preceding the discovery of America. Of
late years there has been discovered in your Western Territories
that marvellous accumulation of deposits, admirably adapted for the
preservation of organic remains, to which I referred the other evening,
and which furnishes us with a consecutive series of records of the fauna
of the older half of the Tertiary epoch, for which we have no
parallel in Europe. They have yielded fossils in an excellent state of
conservation and in unexampled number and variety. The researches of
Leidy and others have shown that forms allied to the _Hipparion_ and
the _Anchitherium_ are to be found among these remains. But it is only
recently that the admirably conceived and most thoroughly and patiently
worked-out investigations of Professor Marsh have given us a just idea
of the vast fossil wealth, and of the scientific importance, of these
deposits. I have had the advantage of glancing over the collections in
Yale Museum; and I can truly say that, so far as my knowledge extends,
there is no collection from any one region and series of strata
comparable, for extent, or for the care with which the remains have
been got together, or for their scientific importance, to the series of
fossils which he has deposited there. This vast collection has yielded
evidence bearing upon the question of the pedigree of the horse of the
most striking character. It tends to show that we must look to America,
rather than to Europe, for the original seat of the equine series;
and that the archaic forms and successive modifications of the horse's
ancestry are far better preserved here than in Europe.
Professor Marsh's kindness has enabled me to put before you a diagram,
every figure in which is an actual representation of some specimen which
is to be seen at Yale at this present time (Fig. 9).
Fig. 9.
The succession of forms which he has brought together carries us from
the top to the bottom of the Tertiaries. Firstly, there is the
true horse. Next we have the American Pliocene form of the horse
(_Pliohippus_); in the conformation of its limbs it presents some
very slight deviations from the ordinary horse, and the crowns of
the grinding teeth are shorter. Then comes the _Protohippus,_ which
represents the European _Hipparion,_ having one large digit and two
small ones on each foot, and the general characters of the fore-arm and
leg to which I have referred. But it is more valuable than the
European _Hipparion_ for the reason that it is devoid of some of the
peculiarities of that form--peculiarities which tend to show that the
European _Hipparion_ is rather a member of a collateral branch, than a
form in the direct line of succession. Next, in the backward order
in time, is the _Miohippus,_ which corresponds pretty nearly with the
_Anchitherium_ of Europe. It presents three complete toes--one large
median and two smaller lateral ones; and there is a rudiment of that
digit, which answers to the little finger of the human hand.
The European record of the pedigree of the horse stops here; in the
American Tertiaries, on the contrary, the series of ancestral equine
forms is continued into the Eocene formations. An older Miocene form,
termed _Mesohippus,_ has three toes in front, with a large splint-like
rudiment representing the little finger; and three toes behind. The
radius and ulna, the tibia and the fibula, are distinct, and the short
crowned molar teeth are anchitherioid in pattern.
But the most important discovery of all is the _Orohippus,_ which
comes from the Eocene formation, and is the oldest member of the equine
series, as yet known. Here we find four complete toes on the front limb,
three toes on the hind limb, a well-developed ulna, a well-developed
fibula, and short-crowned grinders of simple pattern.
Thus, thanks to these important researches, it has become evident that,
so far as our present knowledge extends, the history of the horse-type
is exactly and precisely that which could have been predicted from
a knowledge of the principles of evolution. And the knowledge we now
possess justifies us completely in the anticipation, that when the still
lower Eocene deposits, and those which belong to the Cretaceous epoch,
have yielded up their remains of ancestral equine animals, we shall
find, first, a form with four complete toes and a rudiment of the
innermost or first digit in front, with, probably, a rudiment of the
fifth digit in the hind foot; [4] while, in still older forms, the series
of the digits will be more and more complete, until we come to the
five-toed animals, in which, if the doctrine of evolution is well
founded, the whole series must have taken its origin.
That is what I mean by demonstrative evidence of evolution. An inductive
hypothesis is said to be demonstrated when the facts are shown to be in
entire accordance with it. If that is not scientific proof, there are
no merely inductive conclusions which can be said to be proved. And the
doctrine of evolution, at the present time, rests upon exactly as secure
a foundation as the Copernican theory of the motions of the heavenly
bodies did at the time of its promulgation. Its logical basis is
precisely of the same character--the coincidence of the observed facts
with theoretical requirements.
The only way of escape, if it be a way of escape, from the conclusions
which I have just indicated, is the supposition that all these different
equine forms have been created separately at separate epochs of time;
and, I repeat, that of such an hypothesis as this there neither is, nor
can be, any scientific evidence; and, assuredly, so far as I know, there
is none which is supported, or pretends to be supported, by evidence
or authority of any other kind. I can but think that the time will come
when such suggestions as these, such obvious attempts to escape the
force of demonstration, will be put upon the same footing as the
supposition made by some writers, who are I believe not completely
extinct at present, that fossils are mere simulacra, are no indications
of the former existence of the animals to which they seem to belong;
but that they are either sports of nature, or special creations,
intended--as I heard suggested the other day--to test our faith.
In fact, the whole evidence is in favour of evolution, and there is none
against it. And I say this, although perfectly well aware of the
seeming difficulties which have been built up upon what appears to the
uninformed to be a solid foundation. I meet constantly with the argument
that the doctrine of evolution cannot be well founded, because it
requires the lapse of a very vast period of time; while the duration of
life upon the earth thus implied is inconsistent with the conclusions
arrived at by the astronomer and the physicist. I may venture to say
that I am familiar with those conclusions, inasmuch as some years ago,
when President of the Geological Society of London, I took the liberty
of criticising them, and of showing in what respects, as it appeared to
me, they lacked complete and thorough demonstration. But, putting that
point aside, suppose that, as the astronomers, or some of them, and some
physical philosophers, tell us, it is impossible that life could have
endured upon the earth for as long a period as is required by the
doctrine of evolution--supposing that to be proved--I desire to be
informed, what is the foundation for the statement that evolution does
require so great a time? The biologist knows nothing whatever of the
amount of time which may be required for the process of evolution. It
is a matter of fact that the equine forms which I have described to you
occur, in the order stated, in the Tertiary formations. But I have not
the slightest means of guessing whether it took a million of years, or
ten millions, or a hundred millions, or a thousand millions of years,
to give rise to that series of changes. A biologist has no means of
arriving at any conclusion as to the amount of time which may be needed
for a certain quantity of organic change. He takes his time from the
geologist. The geologist, considering the rate at which deposits are
formed and the rate at which denudation goes on upon the surface of the
earth, arrives at more or less justifiable conclusions as to the time
which is required for the deposit of a certain thickness of rocks; and
if he tells me that the Tertiary formations required 500,000,000 years
for their deposit, I suppose he has good ground for what he says, and
I take that as a measure of the duration of the evolution of the horse
from the _Orohippus_ up to its present condition. And, if he is right,
undoubtedly evolution is a very slow process, and requires a great
deal of time. But suppose, now, that an astronomer or a physicist--for
instance, my friend Sir William Thomson--tells me that my geological
authority is quite wrong; and that he has weighty evidence to show
that life could not possibly have existed upon the surface of the earth
500,000,000 years ago, because the earth would have then been too hot to
allow of life, my reply is: "That is not my affair; settle that with
the geologist, and when you have come to an agreement among yourselves
I will adopt your conclusion." We take our time from the geologists and
physicists; and it is monstrous that, having taken our time from the
physical philosopher's clock, the physical philosopher should turn round
upon us, and say we are too fast or too slow. What we desire to know is,
is it a fact that evolution took place? As to the amount of time which
evolution may have occupied, we are in the hands of the physicist and
the astronomer, whose business it is to deal with those questions.
I have now, ladies and gentlemen, arrived at the conclusion of the task
which I set before myself when I undertook to deliver these lectures.
My purpose has been, not to enable those among you who have paid no
attention to these subjects before, to leave this room in a condition
to decide upon the validity or the invalidity of the hypothesis of
evolution; but I have desired to put before you the principles upon
which all hypotheses respecting the history of Nature must be judged;
and furthermore, to make apparent the nature of the evidence and the
amount of cogency which is to be expected and may be obtained from it.
To this end, I have not hesitated to regard you as genuine students and
persons desirous of knowing the truth. I have not shrunk from taking
you through long discussions, that I fear may have sometimes tried
your patience; and I have inflicted upon you details which were
indispensable, but which may well have been wearisome. But I shall
rejoice--I shall consider that I have done you the greatest service
which it was in my power to do--if I have thus convinced you that the
great question which we have been discussing is not one to be dealt with
by rhetorical flourishes, or by loose and superficial talk; but that it
requires the keen attention of the trained intellect and the patience of
the accurate observer.
When I commenced this series of lectures, I did not think it necessary
to preface them with a prologue, such as might be expected from a
stranger and a foreigner; for during my brief stay in your country, I
have found it very hard to believe that a stranger could be possessed
of so many friends, and almost harder that a foreigner could express
himself in your language in such a way as to be, to all appearance, so
readily intelligible. So far as I can judge, that most intelligent, and
perhaps, I may add, most singularly active and enterprising body, your
press reporters, do not seem to have been deterred by my accent from
giving the fullest account of everything that I happen to have said.
But the vessel in which I take my departure to-morrow morning is even
now ready to slip her moorings; I awake from my delusion that I am other
than a stranger and a foreigner. I am ready to go back to my place and
country; but, before doing so, let me, by way of epilogue, tender to you
my most hearty thanks for the kind and cordial reception which you have
accorded to me; and let me thank you still more for that which is
the greatest compliment which can be afforded to any person in my
position--the continuous and undisturbed attention which you have
bestowed upon the long argument which I have had the honour to lay
before you.